This invention relates generally to the field of communication systems and more particularly to trunked radio frequency communication systems with priority channel scan.
In a communication system, messages are often classified as priority or non-priority according to the relative importance of the content of the messages or the relative importance of the person(s) originating the messages. When remote transceiver units are communicating non-priority messages, it is desirable to interrupt this communication when a priority message is generated and to switch immediately to a priority channel where the priority message can be received.
In conventional non-trunked communication systems, "priority scan" is accomplished by the use of a dedicated priority channel. Remote units periodically sample the priority channel for activity, even while they are communicating on a non-priority channel. When activity is sensed on the priority channel, the remote units switch to the priority channel and remain there for the duration of the priority message.
There are several disadvantages with a dedicated priority channel communication system. First, a dedicated priority channel is an inefficient use of the radio frequency spectrum because the priority channel is idle most of the time. Second, when several different groups are using the same system, a priority transmission by one group ties up the priority channel, thereby preventing other groups from transmitting priority messages.
In a trunked system, the remote units typically comprise mobile units, portable units or control stations. Generally, a mobile unit is a transceiving device designed to be installed in vehicles. A portable device is typically understood to be a transceiving device designed to be carried on or about the person. A control station is typically a permanent or semipermanent installation in a building or other fixed location. As used herein, all of these remote transceiving devices are collectively referred to as subscriber units.
Generally, it is known for a subscriber unit participating in a voice conversation to transmit a sub-audible connect tone to the central station (or system controller). The central station, in turn, transmit a digital connect word commonly referred to as the low speed handshake (LSH). Failure of the central station to receive the inbound connect tone will cause the central to reassign the channel to another requesting subscriber or to the pool of available frequencies. Failure of the subscribers to receive the LSH from the central causes them to return to the control channel to await a new channel grant or to request a communication channel.
In prior trunked communication systems, a subscriber unit requests a user channel assignment by transmitting the request to a system controller on an inbound control channel. The controller responds by transmitting a code containing a user channel assignment on an outbound control channel. In addition to the user channel assignment, the code also specifies which group of subscriber units should move to the newly assigned user channel. All subscribers monitoring the control channel and within the specified group then switch over to the newly assigned user channel.
The disadvantage of such a trunked communication system is that once the subscribers switch to an assigned user channel, they can no longer monitor the control channel and consequently will miss commands from the system controller to switch to a priority channel. Subscribers operating on an assigned user channel must wait until the non-priority message is completed before returning to the control channel to receive further channel assignments. Depending on the duration of the non-priority message, the subscribers have usually missed the entire priority message by the time they return to the control channel.